Internal combustion engines, including diesel, gasoline, gaseous fuel, biofuel and other engines known in the art, exhaust a complex mixture of air pollutants. These air pollutants are composed of gaseous compounds, such as, for example, oxides of nitrogen (NOx). Due to increased environmental awareness, more stringent regulations now limit NOx emissions based on engine type, size, and class.
Engine manufacturers can use Selective Catalytic Reduction (SCR) to reduce emission levels of some air pollutants. SCR operates by adding a gaseous or liquid reductant, often urea, to an engine's exhaust gas upstream of a catalyst. Reductant can react with NOx on the catalyst to form H2O and N2, thereby lowering NOx emissions.
Although SCR can reduce emission levels, reductant storage can be problematic at cold or hot temperatures. For example, urea typically freezes at about −10° C. and can degrade above about 60° C. Traditionally, an electric heater can be used to thaw frozen urea. To prevent overheating, urea storage tanks can be placed away from heat sources, such as, engine blocks or exhaust systems. However, temperature sensors, processors, or other control mechanisms are required to maintain urea within a suitable temperature range.
One system designed to maintain reductant temperature within a suitable range is described in U.S. Patent Application Publication No. 2007/0092413 (hereinafter “the '413 publication”) of Hirata et al., published on Apr. 26, 2007. The '413 publication discloses a reducing agent heating system, wherein the reducing agent is injected into an exhaust stream to reduce NOx emissions. The heating system uses heat supplied by a flow of engine coolant, and a heat exchanger transfers heat from the coolant fluid to an injection nozzle or tubing containing the reducing agent. A processor coupled to temperature sensors controls coolant flow via an electronic valve, maintaining the reducing agent's temperature within an appropriate range.
The temperature control system of the '413 publication is complex and expensive. The injection nozzle or tubing must be modified to accommodate a heat exchanger, temperature sensors are required at various locations, and additional computational processing is required to monitor temperatures and regulate coolant flow via the electronic valve. Failure of a single component could freeze or overheat the reducing agent.
The present disclosure is directed at overcoming one or more of the limitations in the prior art.